ext: gecko: Add Silabs Gecko SDK for EFR32FG1P SoCs

This includes:
* the Gecko SDK files for the EFR32FG1P SoCs

Origin: Silicon Labs Gecko SDK
URL: https://github.com/SiliconLabs/Gecko_SDK


Version: v5.1.2 (SHA: 938464c68e6c3b2237388a692f767bb0767ec010)
Purpose: Add support for Silicon Labs EXX32 SoCs
License: Zlib
Maintained-by: External

To update the Gecko SDK, download the current version from the given URL
and replace the following folders:
* Gecko_SDK/platform/Device/ should replace ext/hal/silabs/gecko/Device/
* Gecko_SDK/platform/emlib/ should replace ext/hal/silabs/gecko/emlib/

Signed-off-by: Christian Taedcke <hacking@taedcke.com>